The present invention relates to the field of optical fiber power roadband light sources, and more particularly concerns a superfluorescent ptical fiber source.
Superfluorescent fiber sources are in large demand to make optical sensors (gyroscopes) or for the characterization of DWDM (Dense Wavelength Division Multiplexing) optical communication components such as fiber Bragg gratings and wavelength multiplexers/demultiplexers. Advantageously, the use of a broadband superfluorescent fiber source as a probe signal in characterizing such systems makes it possible to probe the whole communication transmission band at once, thereby allowing to characterize multiple channels simultaneously. For rare-earth-doped fiber amplifiers, the communication transmission band is defined most of the time by the gain spectrum of the gain medium used, and a superfluorescent fiber source made of the same rare-earth doped fiber will allow to cover the needed wavelength band. One of the requirements for a superfluorescent fiber source used in such an application is that it should be very stable, especially if one wants to have precise characterization parameters, such as insertion loss for example. These sources should also have sufficient power to have a good signal to noise ratio at the detector level.
The main challenge associated with making a superfluorescent light source is to be able to recuperate both the forward and backward propagating ASE (Amplified Spontaneous Emission) signal without creating harmful feedback into the pump laser. Thus, a pump/ASE signals reflection discriminator must be implemented. The initial ASE source configurations that were disclosed, such as in U.S. Pat. No. 3,808,549 (MAURER) and U.S. Pat. No. 4,637,025 (SNITZER et al), were very straightforward and did not take into account the recuperation of the ASE signal of opposite direction to the output. In the first configurations to be presented, the pump feedback was not even considered; these fluorescent light sources thus had very unstable output powers. The idea of eliminating feedback by using a filter within the fiber source configuration was only introduced later, but most of the time this function was not combined with the recuperation of the ASE signal of opposite direction to the output, as for example in the device disclosed in U.S. Pat. No. 5,319,652 (MOELLER et al). As well, U.S. Pat. No. 4,938,556 (DIGONNET et al) discloses two configurations that involve a pump/ASE signals reflection discriminator, but both configurations necessitate high cost dichroic (wavelength selective) filters.
There is therefore a need for a superfluorescent source provided with discriminating means to recuperate backward propagating ASE radiation that is low cost, and uses only readily available optical components that are easy to assemble and do not require any alignment.
The present invention therefore provides a superfluorescent fiber source. The superfluorescent source first includes an optical pump source for generating pump radiation, and a length of optical fiber having forward and backward directions. A region in the length of optical fiber defines a gain medium.
A first optical coupler is provided, for coupling the optical pump source to the length of optical fiber, at a point forward of the gain medium. The pump radiation thereby propagates in the backward direction in the gain medium, and is partly absorbed thereby to stimulate the emission of both forward and backward propagating superfluorescent radiation. Residual pump radiation propagates in the backward direction. The first optical coupler selectively transmits the pump radiation in the backward direction, and the superfluorescent radiation in the forward direction.
Also provided is a second optical coupler, for coupling the residual pump radiation out of the length of optical fiber. The second optical coupler is disposed in the length of optical fiber backward of the gain medium, and it selectively transmits the backward propagating superfluorescent radiation along a first path and the residual pump radiation along a second path.
The superfluorescent fiber source also includes reflecting means disposed in the first path, for reflecting the backward propagating superfluorescent radiation in the forward direction.
Finally, absorbing means are provided, disposed in the second path, for absorbing the residual pump radiation.
Advantageously, the present invention provides a simple, reliable superfluorescent fiber source configuration enabling optimal and stable output power. This configuration necessitates only low-cost, readily available optical components which are easy to assemble and do not require any alignment.
The present invention and its advantages will be better understood upon reading the following non-restrictive description of preferred embodiments thereof, made with reference to the accompanying drawings.